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<h1 class="title">BitTorrent extension for arbitrary DHT store</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Version:</th>
<td>2.0.11</td></tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title">Table of contents</p>
<ul class="simple">
<li><a class="reference internal" href="#terminology" id="toc-entry-1">terminology</a></li>
<li><a class="reference internal" href="#messages" id="toc-entry-2">messages</a></li>
<li><a class="reference internal" href="#immutable-items" id="toc-entry-3">immutable items</a><ul>
<li><a class="reference internal" href="#put-message" id="toc-entry-4">put message</a></li>
<li><a class="reference internal" href="#get-message" id="toc-entry-5">get message</a></li>
</ul>
</li>
<li><a class="reference internal" href="#mutable-items" id="toc-entry-6">mutable items</a><ul>
<li><a class="reference internal" href="#put-message-1" id="toc-entry-7">put message</a></li>
<li><a class="reference internal" href="#cas" id="toc-entry-8">CAS</a></li>
<li><a class="reference internal" href="#response" id="toc-entry-9">response</a></li>
<li><a class="reference internal" href="#errors" id="toc-entry-10">errors</a></li>
<li><a class="reference internal" href="#get-message-1" id="toc-entry-11">get message</a></li>
</ul>
</li>
<li><a class="reference internal" href="#signature-verification" id="toc-entry-12">signature verification</a></li>
<li><a class="reference internal" href="#expiration" id="toc-entry-13">expiration</a></li>
<li><a class="reference internal" href="#test-vectors" id="toc-entry-14">test vectors</a><ul>
<li><a class="reference internal" href="#test-1-mutable" id="toc-entry-15">test 1 (mutable)</a></li>
<li><a class="reference internal" href="#test-2-mutable-with-salt" id="toc-entry-16">test 2 (mutable with salt)</a></li>
<li><a class="reference internal" href="#test-3-immutable" id="toc-entry-17">test 3 (immutable)</a></li>
</ul>
</li>
<li><a class="reference internal" href="#resources" id="toc-entry-18">resources</a></li>
</ul>
</div>
<p>This is a proposal for an extension to the BitTorrent DHT to allow
storing and retrieving of arbitrary data.</p>
<p>It supports both storing <em>immutable</em> items, where the key is
the SHA-1 hash of the data itself, and <em>mutable</em> items, where
the key is the public key of the key pair used to sign the data.</p>
<p>There are two new proposed messages, <tt class="docutils literal">put</tt> and <tt class="docutils literal">get</tt>.</p>
<div class="section" id="terminology">
<h1>terminology</h1>
<p>In this document, a <em>storage node</em> refers to the node in the DHT to which
an item is being announced and stored on. A <em>requesting node</em> refers to
a node which makes look-ups in the DHT to find the storage nodes, to
request items from them, and possibly re-announce those items to keep them
alive.</p>
</div>
<div class="section" id="messages">
<h1>messages</h1>
<p>The proposed new messages <tt class="docutils literal">get</tt> and <tt class="docutils literal">put</tt> are similar to the existing
<tt class="docutils literal">get_peers</tt> and <tt class="docutils literal">announce_peer</tt>.</p>
<p>Responses to <tt class="docutils literal">get</tt> should always include <tt class="docutils literal">nodes</tt> and <tt class="docutils literal">nodes6</tt>. Those
fields have the same semantics as in its <tt class="docutils literal">get_peers</tt> response. It should also
include a write token, <tt class="docutils literal">token</tt>, with the same semantics as int <tt class="docutils literal">get_peers</tt>.
The write token MAY be tied specifically to the key which <tt class="docutils literal">get</tt> requested.
i.e. the <tt class="docutils literal">token</tt> can only be used to store values under that one key. It may
also be tied to the node ID and IP address of the requesting node.</p>
<p>The <tt class="docutils literal">id</tt> field in these messages has the same semantics as the standard DHT
messages, i.e. the node ID of the node sending the message, to maintain the
structure of the DHT network.</p>
<p>The <tt class="docutils literal">token</tt> field also has the same semantics as the standard DHT message
<tt class="docutils literal">get_peers</tt> and <tt class="docutils literal">announce_peer</tt>, when requesting an item and to write an
item respectively.</p>
<p>The <tt class="docutils literal">k</tt> field is the 32 byte ed25519 public key, which the signature can be
authenticated with. When looking up a mutable item, the <tt class="docutils literal">target</tt> field MUST be
the SHA-1 hash of this key concatenated with the <tt class="docutils literal">salt</tt>, if present.</p>
<p>The distinction between storing mutable and immutable items is the inclusion of
a public key, a sequence number, signature and an optional salt (<tt class="docutils literal">k</tt>, <tt class="docutils literal">seq</tt>,
<tt class="docutils literal">sig</tt> and <tt class="docutils literal">salt</tt>).</p>
<p><tt class="docutils literal">get</tt> requests for mutable items and immutable items cannot be distinguished
from each other. An implementation can either store mutable and immutable items
in the same hash table internally, or in separate ones and potentially do two
lookups for <tt class="docutils literal">get</tt> requests.</p>
<p>The <tt class="docutils literal">v</tt> field is the <em>value</em> to be stored. It is allowed to be any bencoded
type (list, dict, string or integer). When it's being hashed (for verifying its
signature or to calculate its key), its flattened, bencoded, form is used. It is
important to use the verbatim bencoded representation as it appeared in the
message. decoding and then re-encoding bencoded structures is not necessarily an
identity operation.</p>
<p>Storing nodes MAY reject <tt class="docutils literal">put</tt> requests where the bencoded form of <tt class="docutils literal">v</tt> is
longer than 1000 bytes. In other words, it's not safe to assume storing more
than 1000 bytes will succeed.</p>
</div>
<div class="section" id="immutable-items">
<h1>immutable items</h1>
<p>Immutable items are stored under their SHA-1 hash, and since they cannot be
modified, there is no need to authenticate the origin of them. This makes
immutable items simple.</p>
<p>A node making a lookup SHOULD verify the data it receives from the network, to
verify that its hash matches the target that was looked up.</p>
<div class="section" id="put-message">
<h2>put message</h2>
<p>Request:</p>
<pre class="literal-block">
{
        &quot;a&quot;:
        {
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;v&quot;: <em>&lt;any bencoded type, whose encoded size &lt;= 1000&gt;</em>
        },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;q&quot;,
        &quot;q&quot;: &quot;put&quot;
}
</pre>
<p>Response:</p>
<pre class="literal-block">
{
        &quot;r&quot;: { &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em> },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;r&quot;,
}
</pre>
</div>
<div class="section" id="get-message">
<h2>get message</h2>
<p>Request:</p>
<pre class="literal-block">
{
        &quot;a&quot;:
        {
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;target&quot;: <em>&lt;SHA-1 hash of item (string)&gt;</em>,
        },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;q&quot;,
        &quot;q&quot;: &quot;get&quot;
}
</pre>
<p>Response:</p>
<pre class="literal-block">
{
        &quot;r&quot;:
        {
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;token&quot;: <em>&lt;write token (string)&gt;</em>,
                &quot;v&quot;: <em>&lt;any bencoded type whose SHA-1 hash matches 'target'&gt;</em>,
                &quot;nodes&quot;: <em>&lt;IPv4 nodes close to 'target'&gt;</em>,
                &quot;nodes6&quot;: <em>&lt;IPv6 nodes close to 'target'&gt;</em>
        },
        &quot;t&quot;: <em>&lt;transaction-id&gt;</em>,
        &quot;y&quot;: &quot;r&quot;,
}
</pre>
</div>
</div>
<div class="section" id="mutable-items">
<h1>mutable items</h1>
<p>Mutable items can be updated, without changing their DHT keys. To authenticate
that only the original publisher can update an item, it is signed by a private
key generated by the original publisher. The target ID mutable items are stored
under is the SHA-1 hash of the public key (as it appears in the <tt class="docutils literal">put</tt>
message).</p>
<p>In order to avoid a malicious node to overwrite the list head with an old
version, the sequence number <tt class="docutils literal">seq</tt> must be monotonically increasing for each
update, and a node hosting the list node MUST not downgrade a list head from a
higher sequence number to a lower one, only upgrade. The sequence number SHOULD
not exceed <tt class="docutils literal">MAX_INT64</tt>, (i.e. <tt class="docutils literal">0x7fffffffffffffff</tt>. A client MAY reject any
message with a sequence number exceeding this. A client MAY also reject any
message with a negative sequence number.</p>
<p>The signature is a 64 byte ed25519 signature of the bencoded sequence number
concatenated with the <tt class="docutils literal">v</tt> key. e.g. something like this:</p>
<pre class="literal-block">
3:seqi4e1:v12:Hello world!
</pre>
<p>If the <tt class="docutils literal">salt</tt> key is present and non-empty, the salt string must be included
in what's signed. Note that if <tt class="docutils literal">salt</tt> is specified and an empty string, it is
as if it was not specified and nothing in addition to the sequence number and
the data is signed. The salt string may not be longer than 64 bytes.</p>
<p>When a salt is included in what is signed, the key <tt class="docutils literal">salt</tt> with the value of
the key is prepended in its bencoded form. For example, if <tt class="docutils literal">salt</tt> is &quot;foobar&quot;,
the buffer to be signed is:</p>
<pre class="literal-block">
4:salt6:foobar3:seqi4e1:v12:Hello world!
</pre>
<div class="section" id="put-message-1">
<h2>put message</h2>
<p>Request:</p>
<pre class="literal-block">
{
        &quot;a&quot;:
        {
                &quot;cas&quot;: <em>&lt;optional expected seq-nr (int)&gt;</em>,
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;k&quot;: <em>&lt;ed25519 public key (32 bytes string)&gt;</em>,
                &quot;salt&quot;: <em>&lt;optional salt to be appended to &quot;k&quot; when hashing (string)&gt;</em>
                &quot;seq&quot;: <em>&lt;monotonically increasing sequence number (integer)&gt;</em>,
                &quot;sig&quot;: <em>&lt;ed25519 signature (64 bytes string)&gt;</em>,
                &quot;token&quot;: <em>&lt;write-token (string)&gt;</em>,
                &quot;v&quot;: <em>&lt;any bencoded type, whose encoded size &lt; 1000&gt;</em>
        },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;q&quot;,
        &quot;q&quot;: &quot;put&quot;
}
</pre>
<p>Storing nodes receiving a <tt class="docutils literal">put</tt> request where <tt class="docutils literal">seq</tt> is lower than or equal
to what's already stored on the node, MUST reject the request. If the sequence
number is equal, and the value is also the same, the node SHOULD reset its
timeout counter.</p>
<p>If the sequence number in the <tt class="docutils literal">put</tt> message is lower than the sequence number
associated with the currently stored value, the storing node MAY return an error
message with code 302 (see error codes below).</p>
<p>Note that this request does not contain a target hash. The target hash under
which this blob is stored is implied by the <tt class="docutils literal">k</tt> argument. The key is the SHA-1
hash of the key (<tt class="docutils literal">k</tt>).</p>
<p>In order to support a single key being used to store separate items in the DHT,
an optional <tt class="docutils literal">salt</tt> can be specified in the <tt class="docutils literal">put</tt> request of mutable items.
If the salt entry is not present, it can be assumed to be an empty string, and
its semantics should be identical as specifying a salt key with an empty string.
The salt can be any binary string (but probably most conveniently a hash of
something). This string is appended to the key, as specified in the <tt class="docutils literal">k</tt> field,
when calculating the key to store the blob under (i.e. the key <tt class="docutils literal">get</tt> requests
specify to retrieve this data).</p>
<p>This lets a single entity, with a single key, publish any number of unrelated
items, with a single key that readers can verify. This is useful if the
publisher doesn't know ahead of time how many different items are to be
published. It can distribute a single public key for users to authenticate the
published blobs.</p>
<p>Note that the salt is not returned in the response to a <tt class="docutils literal">get</tt> request. This
is intentional. When issuing a <tt class="docutils literal">get</tt> request for an item is expected to
know what the salt is (because it is part of what the target ID that is being
looked up is derived from). There is no need to repeat it back for bystanders
to see.</p>
</div>
<div class="section" id="cas">
<h2>CAS</h2>
<p>CAS is short for <em>compare and swap</em>, it has similar semantics as CAS CPU
instructions. It is used to avoid race conditions when multiple nodes are
writing to the same slot in the DHT.</p>
<p>The <tt class="docutils literal">cas</tt> field is optional. If present it specifies the sequence number of
the data blob being overwritten by the put. When present, the storing node
MUST compare this number to the current sequence number it has stored under
this key. Only if the <tt class="docutils literal">cas</tt> matches the stored sequence number is the put
performed. If it mismatches, the store fails and an error is returned.
See <a class="reference internal" href="#errors">errors</a> below.</p>
<p>The <tt class="docutils literal">cas</tt> field only applies to mutable puts. If there is no current
value, the <tt class="docutils literal">cas</tt> field SHOULD be ignored.</p>
<p>When sending a <tt class="docutils literal">put</tt> request to a node that did not return any data for the
<tt class="docutils literal">get</tt>, the <tt class="docutils literal">cas</tt> field SHOULD NOT be included.</p>
</div>
<div class="section" id="response">
<h2>response</h2>
<p>Response:</p>
<pre class="literal-block">
{
        &quot;r&quot;: { &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em> },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;r&quot;,
}
</pre>
</div>
<div class="section" id="errors">
<h2>errors</h2>
<p>If the store fails for any reason an error message is returned instead of the
message template above, i.e. one where &quot;y&quot; is &quot;e&quot; and &quot;e&quot; is a tuple of
[error-code, message]). Failures include <tt class="docutils literal">cas</tt> mismatches and the sequence
number is outdated.</p>
<p>The error message (as specified by <a class="reference external" href="https://www.bittorrent.org/beps/bep_0005.html">BEP5</a>) looks like this:</p>
<pre class="literal-block">
{
        &quot;e&quot;: [ <em>&lt;error-code (integer)&gt;</em>, <em>&lt;error-string (string)&gt;</em> ],
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;e&quot;,
}
</pre>
<p>In addition to the error codes defined in <a class="reference external" href="https://www.bittorrent.org/beps/bep_0005.html">BEP5</a>, this specification defines
some additional error codes.</p>
<table border="1" class="docutils">
<colgroup>
<col width="29%" />
<col width="71%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">error-code</th>
<th class="head">description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>205</td>
<td>message (<tt class="docutils literal">v</tt> field)
too big.</td>
</tr>
<tr><td>206</td>
<td>invalid signature</td>
</tr>
<tr><td>207</td>
<td>salt (<tt class="docutils literal">salt</tt> field)
too big.</td>
</tr>
<tr><td>301</td>
<td>the CAS hash mismatched,
re-read value and try
again.</td>
</tr>
<tr><td>302</td>
<td>sequence number less than
current.</td>
</tr>
</tbody>
</table>
<p>An implementation MUST emit 301 errors if the cas mismatches. This is a
critical feature in synchronization of multiple agents sharing an immutable
item.</p>
</div>
<div class="section" id="get-message-1">
<h2>get message</h2>
<p>Request:</p>
<pre class="literal-block">
{
        &quot;a&quot;:
        {
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;target:&quot; <em>&lt;20 byte SHA-1 hash of public key and salt (string)&gt;</em>
        },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;q&quot;,
        &quot;q&quot;: &quot;get&quot;
}
</pre>
<p>Response:</p>
<pre class="literal-block">
{
        &quot;r&quot;:
        {
                &quot;id&quot;: <em>&lt;20 byte id of sending node (string)&gt;</em>,
                &quot;k&quot;: <em>&lt;ed25519 public key (32 bytes string)&gt;</em>,
                &quot;nodes&quot;: <em>&lt;IPv4 nodes close to 'target'&gt;</em>,
                &quot;nodes6&quot;: <em>&lt;IPv6 nodes close to 'target'&gt;</em>,
                &quot;seq&quot;: <em>&lt;monotonically increasing sequence number (integer)&gt;</em>,
                &quot;sig&quot;: <em>&lt;ed25519 signature (64 bytes string)&gt;</em>,
                &quot;token&quot;: <em>&lt;write-token (string)&gt;</em>,
                &quot;v&quot;: <em>&lt;any bencoded type, whose encoded size &lt;= 1000&gt;</em>
        },
        &quot;t&quot;: <em>&lt;transaction-id (string)&gt;</em>,
        &quot;y&quot;: &quot;r&quot;,
}
</pre>
</div>
</div>
<div class="section" id="signature-verification">
<h1>signature verification</h1>
<p>In order to make it maximally difficult to attack the bencoding parser, signing
and verification of the value and sequence number should be done as follows:</p>
<ol class="arabic simple">
<li>encode value and sequence number separately</li>
<li>concatenate (&quot;4:salt&quot; <em>length-of-salt</em> &quot;:&quot; <em>salt</em>) &quot;3:seqi&quot; <em>seq</em>
&quot;e1:v&quot; <em>len</em> &quot;:&quot; and the encoded value.
sequence number 1 of value &quot;Hello World!&quot; would be converted to:
&quot;3:seqi1e1:v12:Hello World!&quot;. In this way it is not possible to convince a
node that part of the length is actually part of the sequence number even if
the parser contains certain bugs. Furthermore it is not possible to have a
verification failure if a bencoding serializer alters the order of entries in
the dictionary. The salt is in parenthesis because it is optional. It is only
prepended if a non-empty salt is specified in the <tt class="docutils literal">put</tt> request.</li>
<li>sign or verify the concatenated string</li>
</ol>
<p>On the storage node, the signature MUST be verified before accepting the store
command. The data MUST be stored under the SHA-1 hash of the public key (as it
appears in the bencoded dict) and the salt (if present).</p>
<p>On the requesting nodes, the key they get back from a <tt class="docutils literal">get</tt> request MUST be
verified to hash to the target ID the lookup was made for, as well as verifying
the signature. If any of these fail, the response SHOULD be considered invalid.</p>
</div>
<div class="section" id="expiration">
<h1>expiration</h1>
<p>Without re-announcement, these items MAY expire in 2 hours. In order
to keep items alive, they SHOULD be re-announced once an hour.</p>
<p>Any node that's interested in keeping a blob in the DHT alive may announce it.
It would simply repeat the signature for a mutable put without having the
private key.</p>
</div>
<div class="section" id="test-vectors">
<h1>test vectors</h1>
<div class="section" id="test-1-mutable">
<h2>test 1 (mutable)</h2>
<p>value:</p>
<pre class="literal-block">
12:Hello World!
</pre>
<p>buffer being signed:</p>
<pre class="literal-block">
3:seqi1e1:v12:Hello World!
</pre>
<p>public key:</p>
<pre class="literal-block">
77ff84905a91936367c01360803104f92432fcd904a43511876df5cdf3e7e548
</pre>
<p>private key:</p>
<pre class="literal-block">
e06d3183d14159228433ed599221b80bd0a5ce8352e4bdf0262f76786ef1c74d
b7e7a9fea2c0eb269d61e3b38e450a22e754941ac78479d6c54e1faf6037881d
</pre>
<p><strong>target ID</strong>:</p>
<pre class="literal-block">
4a533d47ec9c7d95b1ad75f576cffc641853b750
</pre>
<p><strong>signature</strong>:</p>
<pre class="literal-block">
305ac8aeb6c9c151fa120f120ea2cfb923564e11552d06a5d856091e5e853cff
1260d3f39e4999684aa92eb73ffd136e6f4f3ecbfda0ce53a1608ecd7ae21f01
</pre>
</div>
<div class="section" id="test-2-mutable-with-salt">
<h2>test 2 (mutable with salt)</h2>
<p>value:</p>
<pre class="literal-block">
12:Hello World!
</pre>
<p>salt:</p>
<pre class="literal-block">
foobar
</pre>
<p>buffer being signed:</p>
<pre class="literal-block">
4:salt6:foobar3:seqi1e1:v12:Hello World!
</pre>
<p>public key:</p>
<pre class="literal-block">
77ff84905a91936367c01360803104f92432fcd904a43511876df5cdf3e7e548
</pre>
<p>private key:</p>
<pre class="literal-block">
e06d3183d14159228433ed599221b80bd0a5ce8352e4bdf0262f76786ef1c74d
b7e7a9fea2c0eb269d61e3b38e450a22e754941ac78479d6c54e1faf6037881d
</pre>
<p><strong>target ID</strong>:</p>
<pre class="literal-block">
411eba73b6f087ca51a3795d9c8c938d365e32c1
</pre>
<p><strong>signature</strong>:</p>
<pre class="literal-block">
6834284b6b24c3204eb2fea824d82f88883a3d95e8b4a21b8c0ded553d17d17d
df9a8a7104b1258f30bed3787e6cb896fca78c58f8e03b5f18f14951a87d9a08
</pre>
</div>
<div class="section" id="test-3-immutable">
<h2>test 3 (immutable)</h2>
<p>value:</p>
<pre class="literal-block">
12:Hello World!
</pre>
<p><strong>target ID</strong>:</p>
<pre class="literal-block">
e5f96f6f38320f0f33959cb4d3d656452117aadb
</pre>
</div>
</div>
<div class="section" id="resources">
<h1>resources</h1>
<p>Libraries that implement ed25519 DSA:</p>
<ul class="simple">
<li><a class="reference external" href="https://nacl.cr.yp.to/">NaCl</a></li>
<li><a class="reference external" href="https://github.com/jedisct1/libsodium">libsodium</a></li>
<li><a class="reference external" href="https://github.com/nightcracker/ed25519">nightcracker's ed25519</a></li>
</ul>
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